by Donna Leinwand Leger and William M. Welch USA TODAY, USA TODAY

by Donna Leinwand Leger and William M. Welch USA TODAY, USA TODAY

BOSTON -- One of the suspects wanted in Monday's Boston Marathon bombing was shot and killed by police while a second suspect was at large and being pursued, the head of the Massachusetts State Police said early Friday.

Colonel Timothy P. Alben, Superintendent of the Massachusetts State Police said the first suspect from Monday's bombing was shot by police in a gunfight following a pursuit that began Thursday night in Cambridge and ended a short time later in nearby Watertown. He said that suspect died at a hospital.

Alben said the second suspect, seen in photographs distributed by the FBI as wearing a white cap, is still at large. He said people should be on the lookout and that officials consider him a suspect.

"He is dressed in a gray hooded type sweat shirt. He is a light-skinned or Caucasian male with longer brown curly hair,'' Alben said.

Boston Police Commissioner Ed Davis told reporters the man they are seeking is armed and dangerous. A campus police officer at Massachusetts Institute of Technology in Cambridge was shot and killed, and a transit police officer was critically wounded in the pursuit of the suspects.

"We believe this to be a terrorist. We believe this is a man who came here to kill people. We need to get him in custody,' Davis said.

Police did not provide the suspects' names.

Alben said the suspect who died matched the description of the suspect in the marathon bombing case who was shown wearing a black hat in photographs distributed earlier Thursday by the FBI. Alben said the suspect was struck during the firefight and also run over by a car.

He was pronounced dead at Beth Israel Deaconness Medical Center. Doctors there said the suspect was in cardiac arrest when he arrived, with multiple gunshot wounds and trauma on his trunk from an explosion.

The second suspect fled from the car on foot, Alben said. Police are searching the neighborhood for him and have condoned off a 20-block perimeter in Watertown, he said. He urged residents to remain indoors with their doors locked and not to open their doors to anyone but a uniformed police officer, he said. Motorists should not stop for anyone, he said.

The manhunt for the marathon bombing suspects turned to hot pursuit at 10:30 p.m., when the two men robbed a 7-11 convenience store on the MIT campus in Cambridge, Alben said. A few minutes later, police found an MiT campus police officer shot multiple times in his car, Alben said. The police officer died of his injuries at Massachusetts General Hospital.

The suspects then car-jacked a Mercedes SUV, Alben said. Police found the car and the suspects in Watertown, and pursued them into a residential neighborhood where gunfire was exchanged.

Witnesses report hearing between 15 and 50 shots.

Alben said the suspects also threw explosives from the car. Residents, witnesses and media in the area heard at least two large booms.

The Middlesex district attorney's office said in a statement that police responded to reports of an armed carjacking by two males who held a victim at gunpoint for half an hour before being released uninjured.

Police could be seen with guns drawn one one man face down on a paved street. CNN later reported that man was released by police.

The events unfolded overnight as the entire Boston metro area was on high alert following Monday's fatal bomb explosions during the Boston Marathon and as the FBI was leading a massive manhunt for suspects. The developments came on a day when the FBI issued photographs of two men that it said it is seeking and were seen in surveillance video carrying backpacks in the marathon race crowd on Monday before the twin explosions.

"I heard sirens, then a ton of gunshots.,'' said Adam Healy, 31, a behavioral specialist for autism who lives less than a mile from the scene. "And then I heard an explosion amid the gunshots. After the explosion, the sky lit up. "

Dan MacDonald, 40, sitting in a second story Watertown apartment, said he first heard sirens, then gunshots.

"It was about 10 to 15 shots. then there was an onslaught," he said. "There were 25 to 60 shots within 45 seconds. Then the shots stopped and boom. It was like dynamite."